fix Router types, make singleFlight optional

Author: ryansolid

.changeset/wise-owls-do.md

@@ -0,0 +1,5 @@
+---
+"@solidjs/router": patch
+---
+
+fix Router types, make singleFlight optional

src/routers/components.tsx

@@ -31,7 +31,7 @@ export type BaseRouterProps = {
    */
   root?: Component<RouteSectionProps>;
   rootLoad?: RouteLoadFunc;
-  singleFlight: boolean;
+  singleFlight?: boolean;
   children?: JSX.Element | RouteDefinition | RouteDefinition[];
 };